1. Field of the Invention
The present invention relates to new polymers having narrow molecular weight distribution (low Mw/Mn values) and methods for producing such polymers. The invention also relates to aqueous developable photoresist compositions that contain such polymers as a resin binder component. It has been found that use of the polymers as a resist binder provides significantly improved lithographic properties, including enhanced resolution of a patterned resist coating layer.
2. Background
Photoresists are photosensitive films used for transfer of images to a substrate. A coating layer of a photoresist is formed on a substrate and the photoresist layer is then exposed through a photomask to a source of activating radiation. The photomask has areas that are opaque to activating radiation and other areas that are transparent to activating radiation. Exposure to activating radiation provides a photoinduced chemical transformation of the photoresist coating to thereby transfer the pattern of the photomask to the photoresist coated substrate. Following exposure, the photoresist is developed to provide a relief image that permits selective processing of a substrate.
A photoresist can be either positive-acting or negative-acting. For most negative-acting photoresists, those coating layer portions that are exposed to activating radiation polymerize or crosslink in a reaction between a photoactive compound and polymerizable reagents of the photoresist composition. Consequently, the exposed coating portions are rendered less soluble in a developer solution than unexposed portions. For a positive-acting photoresist, exposed portions are rendered more soluble in a developer solution while areas not exposed remain comparatively less developer soluble.
In general, photoresist compositions comprise at least a resin binder component and a photoactive agent. Photoresist compositions are described in Deforest, Photoresist Materials and Processes, McGraw Hill Book Company, New York, ch. 2, 1975 and by Moreay, Semiconductor Lithography, Principles, Practices and Materials, Plenum Press, New York, ch. 2 and 4, both incorporated herein by reference for their teaching of photoresist compositions and methods of making and using the same.
Photoresist resin binders have been produced by several methods, particularly conventional radical, cationic, anionic or condensation processes. However, these prior approaches pose some notable drawbacks.
For example, prior synthetic processes often produce polymers that have a relatively large (e.g., greater than about 2 to 5) molecular weight distribution. Such a large molecular weight distribution can render a resist formulation unsuitable for many commercial applications, particularly high performance applications where highly resolved images of submicron dimensions are required. For example, a large molecular weight distribution can vary the dissolution rate across a resist coating layer which can result in field residues upon development, or otherwise compromise resolution.
Additionally, some prior approaches for synthesis of resist resin binders are inconvenient for large-scale productions. For example, an anionic synthesis is often difficult to control due to impurities such as water or oxygen that are frequently present. Even a small amount of such impurities can have a substantial effect on the polymer synthesis.
It thus would be desirable to have new methods for producing photoresist resin binders, particularly resin binders having a relatively narrow molecular weight distribution (Mw/Mn).